makefiles/docker.inc.mk: export RIOT_CI_BUILD to docker

The variable was not given to docker before and as it is setting
'RIOT_VERSION_OVERRIDE=buildtest' it helps testing in docker more
consistently.

makefiles/docker.inc.mk

@@ -53,6 +53,7 @@ export DOCKER_ENV_VARS = \
   PREFIX \
   QUIET \
   WERROR \
+  RIOT_CI_BUILD \
   RIOT_VERSION \
   SCANBUILD_ARGS \
   SCANBUILD_OUTPUTDIR \